Various silicone compositions have been used for the release paper which prevents sticking and bonding of the substrate such as paper and plastics to a pressure-sensitive adhesive material. Of the silicone compositions, solvent-type silicones have been widely used in view of their release properties and relatively wide choice of the substrate.
However, reduction in the amount of solvent used and recycling of the solvent with no discharge into the exterior are recent demands in view of environmental pollution, safety, hygiene, and the like. With regard to the solvent reduction, use of a solventless silicone is effective. However, homogeneous coating of such solventless silicone on a substrate such as a paper, laminated paper, or plastic film at a thickness of up to 1 μm requires use of an expensive coater and high skill, and change from a solvent-type silicone to a solventless silicone is not generally employable.
Another effective method for reducing the solvent use is use of an emulsion-type silicone. The silicones of such type are already used, and exemplary such emulsion-type silicones include a mixture of an emulsion comprising an organovinyl polysiloxane, a platinum compound, an emulsifier, and water and an emulsion comprising an organohydrogenpolysiloxane, an emulsifier, and water (JP-B S57-53143), the one produced by emulsion polymerization (JP-A S54-52160), and an organovinylsiloxane and an organohydrogensiloxane which have been emulsified by using a particular emulsifier, and which has been further mixed with an emulsion of a platinum compound (JP-A S63-314275).
These emulsion-type silicones can be diluted with water to any desired degree, and the expensive coater and high skill required in the case of the solventless silicones are not required. Another merit is similarity of the process with the solvent-type silicone.
At present, however, the emulsion-type silicones are not widespread because of the demerits that the dispersion medium is water. One such demerit is the high latent heat of vaporization of the water which inevitably results in the curing at a high temperature, namely, lower curability compared to the solvent-type and the solventless silicones. Another major demerit is the large surface tension of the water which results in the inferior wettability of the substrate, and hence, poor adhesion. These demerits are serious particularly in the case of the plastic film substrate, and these are the reason for the rare use of the emulsion-type silicones.
Many improvements have been proposed to solve the problems as described above. Exemplary such improvements include use of an organopolysiloxane having an alkenyl group at the terminal of the molecule (JP-A H06-57144), and blending of an emulsion comprising a non-silicone polymer (JP-A H11-222557). However, many of these improvements are directed to paper substrates, and there has been little or no proposal of the silicone emulsion which exhibits satisfactory adhesion when the emulsion is coated on a plastic film substrate.
In view of the situation as described above, the inventors of the present invention made various investigations and reported that a silicone emulsion composition mainly comprising an organopolysiloxane wherein a trifunctional siloxane unit (T unit) constitutes about 35 to 60% by mole in the entire siloxane units and an alkenyl group constitutes at least 20% by mole in the entire organic groups exhibits good adhesion to the plastic film substrate (JP 3824072). While this composition exhibits good adhesion to various plastic substrates, an organopolysiloxane containing a large amount of trifunctional siloxane units and alkenyl groups constituted at least 50% by weight of this composition as the component realizing the adhesion, and it has been difficult to realize a sufficiently high releasability to acryl pressure-sensitive adhesives.
In the meanwhile, higher releasability tends to be required for the release films used for a plastic film substrate such as those used in optical applications and electronic and electric component applications. However, there has been no silicone emulsion release agent for plastic films which satisfied such demand.